Density, Viscosity,\nand Refractive Index of 1‑Alkyl-2-methylpyrazolium\nMethyl Sulfate and Bis(trifluoromethylsulfonyl)imide Ionic Liquids

In this study, six pure 1-alkyl-2-methylpyrazolium-based\nionic\nliquids (ILs) with methyl sulfate and bis(trifluoromethylsulfonyl)imide\nanions ([C<sub><i>n</i></sub>mpz][MeSO<sub>4</sub>] and\n[C<sub><i>n</i></sub>mpz][NTf<sub>2</sub>], <i>n</i> = 2, 3, and 4) were prepared and characterized. The density, viscosity,\nand refractive index of these pure ILs were measured from 298.15 to\n353.15 K at 101.3 kPa, which fitted well with the corresponding empirical\nequations. Moreover, the values of the isobaric thermal expansion\ncoefficient were also obtained. The results of experiments and density\nfunctional theory calculations indicate that the adjacent positions\nof two nitrogen atoms in pyrazolium cations lead to the close packing\nof ions, strong cation–anion interaction, and high molar polarizability\nper unit volume. The effects of temperature and <i>N</i>-alkyl chain length on the density, viscosity, and refractive index\nof these ILs are similar to those for imidazolium-based salts. Compared\nwith [C<sub><i>n</i></sub>mpz][MeSO<sub>4</sub>], [C<sub><i>n</i></sub>mpz][NTf<sub>2</sub>] exhibits a higher density,\nlower viscosity, and lower refractive index under the same conditions.\nMoreover, the correlation between density and refractive index was\nstudied, and a new empirical equation was proposed to express this\ncorrelation at different temperatures. It is expected that this work\nwill enable deeper insights into the properties, structure, and structure–property\nrelationship of pure pyrazolium-based ILs.